Polysulfone membranes and hollow fiber membranes are known to the art and have been prepared to a variety of specifications. Hollow fiber membrane fabrication may be accomplished via a wet spinning technique as follows. A casting solution containing a polymer in a solvent is brought into contact with a precipitation solution containing a solvent and a non-solvent. As a result, the polymer precipitates and the membrane is formed. The porous structure and performance of the resulting membrane is determined by the polymer precipitation rate, which is governed by the exchange rate between the solvent and non-solvent. This process causes the concentrations of components in the precipitation bath to vary with the spinning time and therefore effects the polymer precipitation rate. As a consequence, the morphology characteristics, as well as the performance of the resulting membrane, will change with the spinning time.
To overcome the problems associated with the wet spinning process using a solvent mixture as a precipitation medium, a large volume precipitation bath may be employed. The large volume precipitation bath minimizes, but does not avoid, the variation in the concentrations of the precipitation medium. Further, the process becomes more difficult when dealing with large volumes due to high levels of solvent vapor, difficulties in mixing the precipitation solution and temperature control. This is particularly problematic for a continuous spinning process where a solvent mixture is used to precipitate the membrane.
In order to overcome the problems associated with the prior art processes, a new process has been developed wherein the concentration of the components in the precipitation solution are kept constant throughout the spinning process.